1. Field of Applicable Technology
The present invention relates to a stamper which is used in producing recording disks having a high density of recorded data, and to a method of manufacture of such a stamper.
2. Description of the Related Art
In recent years, technical advances have been made in the field of recording disks, and in particular in the field of optical recording disks such as CDs (compact disks). Improvements are being envisaged for such optical recording disks whereby a narrower track pitch and increased linear recording density can be achieved, so that for example it will be possible to produce long-playing CDs, or CDs which can reproduce images.
In such optical recording disks, information is recorded in the form of very small depressions, referred to as "pits" which are formed in a surface of a disk made of a plastic material, with that surface then being coated with a thin film of optically reflective material, which is then covered by a protective film. The pits, which correspond to an information signal pattern that is to be reproduced, are formed while the plastic disk is in a soft condition, by being transferred (e.g. by injection molding) from a surface of a stamper which has a pattern formed thereon which is the inverse of the pattern of pits. That is to say, for each pit which is to be formed in the recording surface of a recording disk, there is a corresponding small protrusion on the stamper surface. For clarity of description, these protrusions will be referred to in the following as the stamper protrusions, and the face of the stamper having these protrusions will be referred to as the stamper surface.
The essential factor for achieving an increased recording density of such a recording disk is to reduce both the pit length and the width dimensions. However it is difficult to achieve such a reduction, due to problems in manufacturing a stamper which will accurately form pits having a very small width. In the prior art, the stamper has in general been formed of nickel (Ni). However nickel is a relatively soft metal, and so is not ideally suited to such manufacturing process. In addition, it is essential to achieve an extremely flat surface for the stamper, and nickel is not an ideal material for achieving a highly flat surface.
Glass is harder than metal,and is also superior with regard to achieving a flat surface, and so could in principle be used to manufacture a stamper for forming high-density recording disks. Furthermore if glass were to be used as the material of a stamper, then because of the low thermal conductivity of glass, a problem of bifringence of the recording disks which occurs in the prior art could be reduced. That problem is described in Japanese Patent Application Provisional Publication 3-119534.
A method of producing a stamper formed of glass, for use in forming high-density recording discs has been described by the present applicant in Japanese Patent Application Provisional Publication 1-188332. The essential details of that method of manufacture will be summarized in the following, referring to FIG. 1. In the first step illustrated in diagram (a) of FIG. 1, a layer of negative-type photoresist 2 (i.e. a type of photoresist whereby regions which have been exposed to light can subsequently be left unchanged while other regions are removed by a solvent) is evenly coated over a highly polished upper surface of a quartz glass substrate 1. In the next step illustrated by diagram (b), a focussed laser light beam 3 which is modulated by the information signal pattern that is to be recorded is scanned over the photoresist layer 2 to thereby selectively expose successive portions of the photoresist layer 2 in accordance with the information signal pattern. Specifically, each region where a pit is to be formed on the recording disks (i.e. a protrusion is to be formed on the stamper surface) is exposed to the light beam 3. The photoresist layer 2 is then developed, i.e. the portions of that layer which have not been exposed to light are removed by dissolving in a suitable solvent, leaving the desired pattern of photoresist portions as illustrated in diagram (c). Dry etching of the substrate is then performed by a CF.sub.4 gas plasma 4, with the etching being halted when a desired depth has been reached as illustrated in diagram (d). The remaining portions of photoresist 2 are then removed by an oxygen plasma .to obtain the completed glass stamper, shown in diagram (e), with the protrusions 5 formed on the stamper surface. Using that stamper, disk substrates can then be formed, by a molding method in which a surface of a soft plastic layer has a pattern of pits formed therein by the pattern of stamper protrusions, the plastic then being hardened to form a disk substrate, with the disk substrate being then having a recording film (optically reflective) and a protective film formed thereon, to obtain a high-density optical recording disk.
In the following, the term "pit width" will be used to refer to the width of each stamper protrusion (i.e. the dimension as measured at right angles to a recording track direction along which stamper protrusions are successively formed), as well as to the width of the resultant pits which will be formed in a recording disk by that stamper.
In the above description, the photolithography operation to form the photoresist pattern of diagram (b) is carried out by using a negative-type photoresist, in conjunction with dry etching. However with such a method of photolithography, the minimum pit width that can be achieved is approximately 1.00 .mu.m. Thus for example in the case of a CD, the maximum recording density is limited as a result of the relatively large size of the pit width, and it has not hitherto been possible to manufacture a stamper formed of glass which will produce a smaller pit width. This is due to the fact that even if a shorter wavelength of light is used for exposure in the photolithography process, the pit size cannot be further reduced due to the chemical characteristics of the photoresist material. Specifically, the solubility ratio of the portions of the photoresist which have been exposed to light and the portions of the photoresist which have not been exposed is small. Here, "solubility" is with respect to a solvent that is used to remove the portions of the photoresist which have not been exposed to light, after the step described hereinabove of selectively exposing the photoresist layer to light has been completed.